Devices for reducing vibrations of a structure are not new. For example, vibrations from aircraft engines are commonly transmitted via the wing and fuselage into the passenger cabin thereby generating unwanted noise inside the cabin. A common method for reducing these vibrations is to fasten a device to the vibrating structure which vibrates at the same frequency but out of phase with the vibrating structure.
One conventional type of vibration reducer is a mass-spring having a resonant frequency that equals the vibration frequency of the vibrating structure. In some applications, however, the vibration frequency of the structure varies making it useful to employ a vibration reducer having a variable resonant frequency.
Other conventional vibration reducers have been disclosed such as in U.S. Pat. No. 4,042,230 by Akimoto which discloses a vibration absorber having pneumatic springs that have adjustable internal pressures to vary the frequency of the absorber; and U.S. Pat. No. 3,387,499 by Bruderlein which discloses a vibrator having a rotating unbalanced mass for generating vibrations which have their amplitude damped by electromagnetic means.